Camera system for editing documents

ABSTRACT

A camera system ( 2 ) for strip-shaped exposure of a bank note (BN) having good optical properties as well as a compact construction has, symmetrically to an optical axis (OA), mirror assemblies ( 4   a   , 4   b ) of elliptical cross section each extending in the direction of the illuminated strip, light sources formed by LED arrays ( 6   a   , 6   b ), an imaging optic in the form of a SELFOC® lens assembly ( 8 ), and a photodetector array ( 12 ) onto which the illuminated strip of the bank note (BN) is imaged by the SELFOC® lens assembly ( 8 ) in a 1:1 ratio.

[0001] This invention relates to a camera system for the processing ofdocuments, in particular for reflectance and/or transmittancemeasurement of documents.

[0002] Such a camera system can be used for example for evaluatingdocuments, e.g. bank notes. In usual bank note processing devices, anillumination device having a light source and a mirror assembly is usedto illuminate the document to be judged, and the light reflected by thedocument is directed onto a detector, for example a CCD sensor, toobtain digital images of the document. The image signals can then beevaluated for various purposes.

[0003] Such camera systems are generally formed for reflectance ortransmittance measurement of documents, in particular bank notes. Forthe light reflected by the bank note to be detected as faithfully aspossible, an optic is located in the prior art between the object planetraversed by the bank note and the detector for transferring an image ofthe illuminated area—usually a strip—of the bank note onto the detectorin reduced size. A disadvantage of known camera systems and the opticsused therein is the great distance between object and detector.Furthermore, the reduction in size leads to distortions in the imagegenerated by the optic on the detector, which require correction foruseful image signals to be obtained. The usually employed light sources,for example fluorescent lamps, have only a relatively short life andmust thus be replaced at certain time intervals.

[0004] The invention is based on the problem of stating a camera systemof the above-mentioned kind that is characterized by good opticalproperties while having a compact construction.

[0005] This problem is solved in a camera system with the abovementionedfeatures if the mirror assembly is disposed around the detector, thelight source is formed as an LED array, and the optic is formed for a1:1 image transfer of the illuminated place on the document onto thedetector.

[0006] The mirror assembly built around the detector (for example a CCDsensor) creates the possibility for a compact structure of the camerasystem, whereby the light source formed as an LED array guarantees along life of the illumination device as well as sufficient luminosity ofa suitable wavelength. The 1:1 image transfer permits the optic toprovide an image very largely free from distortions on the detector,thus making elaborate correction unnecessary.

[0007] If, as is preferred, the camera system is used for processingbank notes, in particular examining bank notes, the light-emitting diodearray is formed to emit magenta light, a mixture of wavelengths of redand blue light. Light of said wavelengths causes a strengthening of thegreen contrast, i.e. a strengthening of the color used for executing alarge part of the writing on bank notes. However, a suitable applicationof red, green and blue light-emitting diodes makes it fundamentallypossible to produce any color required for illuminating the bank notesor documents to be examined.

[0008] As is known per se, the total camera system is formed forstrip-shaped illumination of a document and strip-shaped recording ofthe light coming from the document. For examination of documents, thedocuments are moved past before a window of the camera system in anobject plane. In a preferred embodiment of the invention, the mirrorassembly of the illumination device consists in cross section of twoelliptical sections, each elliptical section having an LED arrayassociated therewith. The LED arrays emit light at a certain angle ofradiation. The mirror assembly and the angles of radiation of the LEDscan be coordinated well with each other so that high luminous efficiencyis attained by the LEDs.

[0009] The detector can be for example a CCD sensor, but preferablyconsists of photodetector arrays, which are available on the market asCMOS photodetector chips. Depending on the desired resolution, such CMOSphotodetector chips can be disposed in a larger or smaller number on agiven surface area.

[0010] The inventive camera system is equally suitable for reflectancemeasurement and transmittance measurement as well as for combinedmeasurement, the illumination device used for reflectance measurementbeing also used for transmittance measure-measurement. For reflectancemeasurement, that is, for measuring the light reflected by the document,a detector and the optic are located between the LED arrays and thedocument. For transmittance measurement, an optic and a detector arelocated on the side of the object plane facing away from theillumination device, whereby said devices provided for transmittancemeasurement can be provided additionally or alternatively to the opticand detector for reflectance measurement.

[0011] For transmittance measurement, a dark field sensor can beprovided, on the one hand, and a light field sensor, on the other hand.With the dark field sensor, optic and detector are located at a placeoffset from the transmittance beam path. For light field measurement,optic and detector are located in the beam path of the light that haspassed through the document.

[0012] In almost all documents, the image on one side of the documentdiffers from that on the other side of the document. To obtain extensivedata from the document within a short time, the invention provides tworeflectance measurement devices and two transmittance measurementdevices. It is specially provided to dispose an illumination device withoptic and detector on the first side of the object plane for a firstreflectance measurement and an optic and detector on the second side ofthe object plane for a first transmittance measurement, thetransmittance measurement making use of the illumination deviceassociated with the reflectance measuring device. Such an assembly isprovided once again, offset by 180° (π) from the optical plane. The banknote is then subjected in one run to reflectance measurement on bothsides, furthermore it is subjected to transmittance measurement on bothsides, whereby a light field measurement, on the one hand, and a darkfield measurement, on the other hand, are performed for bothtransmittance measurements.

[0013] A special feature of the invention is the use of an optic thatdelivers a 1:1 image transfer of the illuminated strip of the bank noteonto the detector. Such optics have been known for some time. Apreferred optic for the present invention is one in the form of lensesknown under the trade name “SELFOC®.” SELFOC® lenses are fiber bundleassemblies that deliver an upright 1:1 image of the object.

[0014] The special features of the invention explained above, i.e. anillumination device with an in particular elliptical mirror assembly anddouble LED array, a SELFOC® lens assembly generating 1:1 image transfer,and a photodetector array in the image plane of the lens assembly,obtain a compactly built, easily and cheaply produced camera system withgood optical properties. Since the image arising in the image plane ofthe lens assembly is very largely distortion-free with a 1:1 enlargementwith respect to the object plane, no corrections are required. The LEDarrays have long life; in particular the LEDs can be chosen so as toobtain the most favorable color of illumination light for the particularcase of application.

[0015] The inventive camera system contains components known from theprior art. A peculiarity is the mirror assembly used. Said mirrorassembly can be coordinated with the other components, in particular theLED arrays and the overall depth of the illumination device with respectto the object plane by known calculation methods and software. Themirror surfaces can be milled from metal blocks by numericallycontrolled machines, or else made of mirror-surfaced, transparentmaterial. In the latter case, entrance and exit surfaces can be providedwith a lens made of the transparent material.

[0016] In the following, some examples of the invention will beexplained in more detail with reference to the drawings, in which:

[0017]FIG. 1 shows a sectional view through a first embodiment of aninventive camera system for reflectance measurement of bank notes, thesectional plane extending perpendicular to a plane defined by opticalaxes of an assembly that extends perpendicular to the plane ofprojection with a constant cross section;

[0018]FIG. 2 shows a schematic view of the beam path of an illuminationdevice as used in the camera system shown in FIG. 1;

[0019]FIG. 3 shows a schematic view of a further embodiment of aninventive camera system for two reflectance measurements and twotransmittance measurements; and

[0020]FIG. 4 shows a sectional view of the camera system shown in FIG. 3corresponding to cutting line IV-IV in FIG. 3.

[0021]FIG. 1 shows a first embodiment of camera system 2 according tothe invention. The sectional view shows a camera system for performing areflectance measurement in bank note BN transported in object plane OEfrom the bottom to the top in the direction of the arrow.

[0022] During transport of bank note BN past a window of camera system2, said window being about 0.5 millimeters wide in the transportdirection and not shown in detail here, a strip is illuminated on theleft side of bank note BN in FIG. 1. Light reflected by the bank note isimaged onto a photodetector array. Illumination of bank note BN is doneusing illumination device 14 disposed symmetrically to optical axis OAof the camera system. In the sectional view shown in FIG. 1, opticalaxis OA corresponds to a plane of symmetry perpendicular to the plane ofprojection and containing optical axis OA.

[0023] Illumination device 14 contains mirrors 4 a, 4 b of ellipticalcross section and linearly disposed LED arrays (light-emitting diodearrays) 6 a, 6 b, said mirrors and LED arrays being disposed on eachside of optical axis OA.

[0024] Light concentrated in the form of a strip on bank note BN byillumination device 14 according to the directions of the arrows isreflected by bank note BN and passes into entrance side 8E of SELFOC®lens assembly 8. The light leaves said lens assembly via light exitsurface 8A and is imaged onto the surface of photodetector array 12 oncircuit board 10. The photodetector array is an assembly of CMOSphotodetector chips, which can be disposed on board 10 with selectableresolution. For example, in a low-resolution detector the chips aredisposed on board 10 at a pixel center distance of 1 millimeter, whilefor high resolution the center distance of the pixels of detector 12 ispreferably 0.2 millimeters.

[0025] The drawing does not show the circuit elements for drivingillumination device 14 and detector 12 and for processing the imagesignals obtained from detector 12. Said circuit devices can be formed asin conventional camera systems of a comparable type in a way obvious tothe expert.

[0026] As is evident from FIG. 1, the light-emitting diodes of LEDarrays 6 a and 6 b have certain angles of radiation adapted to mirrors 4a and 4 b having an elliptical cross section, an area free fromillumination light remaining in the center area around optical axis OAfor receiving SELFOC® lens assembly 8 and board 10 with detector 12.Lens assembly 8 formed as a fiber optic system permits a 1:1 imagetransfer of the illuminated strip on bank note BN onto thephotosensitive surface of detector 12. The image is practically freefrom distortions. No correction is required. As one can see, the totalstructure is compact and simple, so that the camera system can beproduced in a compact construction with low production effort. The useof light-emitting diodes in the illumination device guarantees a longlife of the illumination device.

[0027] Suitable choice of the light-emitting diodes of LED arrays 6 aand 6 b permits the wavelength of the light directed onto the documentby illumination device 14 to be optimally coordinated with the nature ofthe document to be tested, here bank note BN. In many bank notes, thewriting is executed mainly in green ink. The best possible contrast istherefore obtained if the light-emitting diodes of LED arrays 6 a and 6b deliver magenta light, i.e. have elements for red and blue light in aratio of red to blue of 1:2, according to the embodiments of theinvention described here.

[0028]FIG. 2 shows schematically the beam path of the illuminationlight. The two LED arrays are disposed at the coordinates x=0 and y (6a)=0.1 and y (6 b)=−0.1 in the grid shown in FIG. 2.

[0029] The cross sections of mirrors 4 a and 4 b are such that the beampencils overlap at the place of illumination in object plane OE, asshown enlarged at the bottom right in FIG. 2. The overlapping beamsresult in a defined, largely constant intensity distribution of lightacross the width of the illuminated strip shown in cross section in FIG.2.

[0030]FIG. 3 shows a further, preferred embodiment of the inventivecamera system. With the camera system shown in FIG. 3, first and secondreflectance measurements and first and second transmittance measurementsare performed directly one after the other with bank note BN runningupward according to the arrow in the drawing.

[0031] The top left in FIG. 3 shows camera system 2 similar to thecamera system shown in FIG. 1 as a partial camera system of a multiplemeasuring assembly.

[0032] The elements shown in FIG. 1 can be readily recognized in partialcamera system 2 on the top left in FIG. 3. The detector here is ahigh-resolution detector designated with reference sign 12 h. In theshown example, detector 12 h in the form of a CMOS photodetector chiparray has a resolution of 0.2 millimeters, that is, the pixels of thechip are disposed at a center distance of 0.2 millimeters.

[0033] Light directed by illumination device 14 of partial camera system2 in a strip shape onto bank note BN for reflectance measurement isreflected onto photodetector 12 h via SELFOC® lens assembly 8 in the wayalready shown in FIG. 1. The photodetector delivers image signals to anevaluation circuit not shown here.

[0034] Furthermore, light from illumination device 14 used forreflectance measurement is partly transmitted through bank note BN. Thisresults in a transmitted light distribution on the side facing away fromillumination device 14 of object plane OE traversed by bank note BN,whereby two intensity peaks of the transmitted light, indicated in theFigure by beam pencils on the right side of bank note BN, are presentdue to the beam path generated by illumination device 14.

[0035] Partial camera system 20 shown on the top right in FIG. 3 is usedfor a second transmittance measurement, here in the form of a dark fieldmeasurement. For dark field measurement, SELFOC® lens assembly 8 b islocated between the beam pencils of the transmitted light for imaginglight from the “dark field” onto the surface of low-resolution detector12 l. Detector 12 l is formed by an array of CMOS photodetector chips,which are disposed on a board here according to a resolution of 1millimeter, that is, at a pixel center distance of 1 millimeter.

[0036] Partial camera systems 2 and 20 shown at the top in FIG. 3 areused for a second reflectance measurement and a second transmittancemeasurement.

[0037] Assembly B comprising partial camera systems 2 and 20 is precededin time and space with respect to the transport direction of bank noteBN by assembly A which is formed similarly to assembly B but offsettherefrom by 180° with respect to object plane OE.

[0038] Assembly A comprises two partial camera systems, namely partialcamera system 2′ for a first reflectance measurement and partial camerasystem 20′ for a first transmittance measurement.

[0039] Partial camera system 2′ is formed identically to partial camerasystem 2 shown on the top left in FIG. 3. The components for partialcamera system 2′ are provided with reference signs with prime symbols.

[0040] Partial camera system 20′ is formed similarly to partial camerasystem 20 but is used, unlike the latter, for light field transmittancemeasurement. Accordingly, SELFOC® lens assembly 8 h is located in thecenter of one beam pencil of transmitted light, the output of saidSELFOC® lens assembly 8 h being faced by low-resolution detector 12 l′.Elements 8 h and 12 l′ correspond to elements 8 b and 12 l of partialcamera system 20 on the top right in FIG. 3, only their location withrespect to the transmitted light is intended for a light fieldmeasurement.

[0041]FIG. 4 shows the structure shown in cross section in FIG. 3 in asectional view according to cutting line IV-IV in FIG. 3. One can seefrom the left to the right LED array 6 a, mirror 4 a, detector 12 h,SELFOC® lens assembly 8, bank note-BN traversing object plane OE (inFIG. 4 bank note BN moves perpendicular out of the plane of projection),SELFOC® lens assembly 8 b disposed to the right of object plane OE, withassociated detector 12 l.

[0042]FIG. 4 shows some beam paths of illumination light, reflectancelight and transmitted light. As one can see, the light distribution inthe area of bank note BN is substantially homogeneous. At the upper andlower edges of bank note BN in FIG. 4 the light begins to grow weaker.On both sides of moving bank note BN, that is, at the top and bottom inFIG. 4, there are reference markings R1 and R2 that are used forgenerating a reference signal for adjusting measurements. Referencemarkings R1 and R2 can be formed e.g. as white reference surfaces. Lightreflected by reference markings R1 and R2 is received by detector 12 hand the reference signal derived therefrom.

1. A camera system for processing documents, in particular forreflectance and/or transmittance measurement of documents, for examplebank notes, having an illumination device (14) for illuminating thedocument (BN) traversing an object plane (OE), comprising a light source(6 a, 6 b); and a mirror assembly (4 a, 4 b) associated with the lightsource for directing light emitted by the light source (6 a, 6 b) ontothe document (BN); an optic (8, 8 b, 8 h, 8′) for imaging lightreflected and/or transmitted by the document (BN) onto an image plane,and a detector (12; 12 h; 12 h′, 12 l, 12 l′) disposed in the imageplane; characterized in that the mirror assembly (4 a, 4 b) is disposedaround the detector (12, 12 h, 12 h′, 12 l, 12 l′), the light source isformed as an LED array (6 a, 6 b), and the optic (8, 8 b, 8 h, 8′) isformed for 1:1 image transfer of the illuminated place on the document(BN) onto the detector.
 2. A camera system according to claim 1,characterized in that the illumination device (14) is formed forstrip-shaped illumination of the document (BN), and the illuminationdevice (14), the optic (8, 8 b, 8 h, 8′) and the detector (12, 12 h, 12h′, 12 l, 12 l′) are disposed symmetrically with respect to a plane thatis perpendicular to the document (BN) and determined by the set ofoptical axes (OA) of the optic.
 3. A camera system according to claim 2,characterized in that the mirror assembly (4 a, 4 b) consists of twoelliptical sections in cross section, and two LED arrays (6 a, 6 b) areprovided each in linear form.
 4. A camera system according to claim 3,characterized in that the detector (12, 12 h, 12 h′, 12 l, 12 l′) andthe optic (8, 8 b, 8 h, 8′) are disposed between the LED arrays (6 a, 6b) and the object plane (OE) for reflectance measurement.
 5. A camerasystem according to claim 3 or 4, characterized in that an optic (8 b, 8h) and a detector (12 l, 12 l′) are disposed on the side of the objectplane (OE) facing away from the illumination device (14) fortransmittance measurement.
 6. A camera system according to claim 5,characterized in that the optic used for transmittance measurement (8 b,8 h) and the corresponding detector (12 l, 12 l′) are disposed in thetransmitted beam path or laterally offset from the transmitted beampath.
 7. A camera system according to claim 5, characterized in that anoptic and a detector are disposed both in the transmitted beam path andlaterally offset therefrom for transmittance measurement.
 8. A camerasystem according to any of claims 2 to 7, characterized in that anillumination device (14′) with optic (8′) and detector (12 h′) isdisposed on the first side of the object plane (OE) for a firstreflectance measurement, and an optic (8 h) and a detector (12 l′) aredisposed on the second side of the object plane (OE) for measuring lightcoming from the illumination device (14′) on the first side for a firsttransmittance measurement, and a further such device (B) rotated by 180°with respect to the object plane is provided for a second reflectancemeasurement and a second transmittance measurement.
 9. A camera systemaccording to claim 8, characterized in that the device (20′) for thefirst transmittance measurement is designed for light field measurementand the device (20) for the second transmittance measurement for darkfield measurement, or vice versa.
 10. A camera system according to anyof claims 1 to 9, characterized in that the optic is formed as a fiberoptic system.
 11. A camera system according to claim 10, characterizedin that the fiber optic system is a SELFOC® lens assembly (8, 8 b, 8 h,8′).
 12. A camera system according to any of claims 1 to 11 for use withbank notes (BN), characterized in that the LED array (6 a, 6 b) isformed to emit magenta light as a mixture of the wavelengths of red andblue light.